Currently, there are several global positioning systems designed to determine geographic location of wireless cellular devices and these systems may use any one of several available techniques to determine geographic position.
Each of these techniques has advantages and disadvantages and the particular technique utilized will depend upon the physical embodiment of the global positioning system used and the desired accuracy. Among these techniques are included an Angle of Arrival/Angle of Incidence technique and a Time of Arrival/Time Difference technique.
Systems which utilize an Angle of Arrival/Angle of Incidence technique to determine the location of a wireless device will include several base stations which are positioned at fixed known locations. In this configuration, the wireless cellular device is configured to transmit a known signal or data pattern at regular time intervals. This signal is received at each base station and the angle of arrival or angle of incidence is determined. Using the angle of arrival at each of the several base stations, the location of the wireless device can be calculated using well known triangulation techniques. This technique, is fairly inexpensive to implement. However, it assumes a direct line of sight between each of the base stations and the wireless device. Accordingly, if the signal from the wireless cellular device has been reflected or redirected in any way before it is received at a base station, the angle of arrival or angle of incidence information for that signal will not accurately reflect the position of the wireless device. Accordingly, this technique is not ideal for determining position of a wireless in areas of heavy foliage or large structures, such as in high rise buildings.
Other global positioning designs use a Time of Arrival Technique in order to determine the location of the wireless cellular device. Systems which utilize a Time of Arrival technique to determine the location of a wireless device also utilize base stations which are positioned at fixed known locations. In a Time of Arrival technique, each base station is configured with its own internal clock. Using this technique, the wireless device is configured to transmit known signals or data patterns at regular time intervals. These signals are received and recognized at each base station. The arrival time at each station is recorded using the internal clock of that station. Based upon the difference in arrival time at each station, the known speed of the signal, and the fixed locations of the stations, the position of the wireless device can be determined using hyberbolic navigational equations.
One draw back to this approach is that the internal clocks in each base station must accurately reflect correct time information in order for the time difference to be accurate. Only when the internal clocks of all base stations are synchronized to indicate the same time information will the time difference of arrival between each base station be accurate.
Each of these systems has been limited to wireless cellular devices, which do not always operate in areas of heavy foliage or large structures, such as in high rise buildings. Accordingly, what is needed is a technique for accurately determining the position of a wireless device using a Time of Arrival technique which will operate indoors.